A Cathode Ray Tube (CRT) display generally provides the best brightness, highest contrast, best color quality and largest viewing angle of prior art displays. CRT displays typically use a layer of phosphor that is deposited on a thin glass faceplate. These CRTs generate a picture by using one to three electron beams that generate electrons which are scanned across the phosphor in a raster pattern. The phosphor converts the electron energy into visible light so as to form the desired picture. However, prior art CRT displays are large and bulky due to the large vacuum bottles that enclose the cathode and extend from the cathode to the faceplate of the display. Therefore, typically, other types of display technologies such as active matrix liquid crystal display, plasma display and electroluminiscent display technologies have been used in the past to form thin displays.
Recently, a thin flat panel display (FPD) has been developed which uses the same process for generating pictures as is used in CRT devices. These flat panel displays use a backplate including a matrix structure of rows and columns of electrodes. One such flat panel display is described in U.S. Pat. No. 5,541,473, titled GRID ADDRESSED FIELD EMISSION CATHODE, by Duboc, Jr. et al., and filed Feb. 1, 1993 which is incorporated herein by reference as background material. Typically, the backplate is formed by depositing a cathode structure (electron emitting) on a glass plate. The cathode structure includes emitters that generate electrons. The backplate typically has an active area within which the cathode structure is deposited. Typically, the active area does not cover the entire surface of the glass plate, leaving a thin strip around the edges of the glass plate. Traces extend through the thin strip to allow for connectivity to the active area.
Prior art flat panel displays include a thin glass faceplate having one or more layers of phosphor deposited over the interior surface thereof. The faceplate is typically separated from the backplate by about 1 millimeter. The faceplate includes an active area within which the layer (or layers) of phosphor is deposited. A thin strip that does not contain phosphor extends from the active area to the edges of the glass plate. The faceplate is attached to the backplate using a glass frit seal. This seal is formed by melting glass frit in a high temperature heating step. This forms an enclosure that is evacuated so as to produce a vacuum between the active area of the backplate and the active area of the faceplate. Individual regions of the cathode are selectively activated to generate electrons which strike the phosphor so as to generate a display within the active area of the faceplate. These flat panel displays have all of the advantages of conventional CRTs but are much thinner.
In prior art fabrication processes, a hollow evacuation tube is placed such that it extends across the thin strip of the backplate. Typically a glass or copper tube is used as the evacuation tube (also referred to as a pump port). A thin layer of glass frit is then deposited around the backplate such that the glass frit surrounds the active area of the backplate. The enclosure is only interrupted by the evacuation tube that extends across a gap in the layer of glass frit.
The faceplate is then placed over the backplate such that the active area of the faceplate is aligned with the active area of the backplate. The resulting flat panel display assembly is then placed in an oven where a high temperature process step is performed so as to melt the frit. The glass frit forms a seal between the faceplate and the backplate as it melts, forming an enclosure into which the evacuation tube extends. Typically, a temperature of about 400 degrees centigrade is required to melt the glass frit.
The flat panel display assembly is then removed from the oven and a vacuum hose is attached to the evacuation tube. Any gas within the enclosure is then removed through the evacuation tube. The evacuation tube is then sealed off and the vacuum hose is removed. The resulting flat panel display has a sealed enclosure which is under a vacuum.
Other prior art processes use an auxiliary chamber for forming a vacuum within the flat panel display. The auxiliary chamber is a structure that is formed on the bottom of the backplate surrounding the opening in the backplate. The auxiliary chamber includes an exhaust port that is typically made of glass. The auxiliary chamber exhaust port is coupled to a vacuum hose for evacuating the gas inside of the flat panel display. An auxiliary chamber is used in conjunction with a backplate which has an opening formed within it. When an auxiliary chamber is used, frit is disposed completely around the circumference of the active area of the backplate. Upon heating the frit, a seal is formed between the faceplate and the backplate that completely encloses the active areas of the faceplate and the active areas of the backplate. The opening in the backplate is disposed within the area enclosed by the frit seal. Once the flat panel display is evacuated, a localized heat source is used to seal the exhaust port.
The sealing process is time consuming and expensive due to the numerous fabrication steps. In addition, such prior art sealing processes subject the entire flat panel display to very high temperatures which are required to melt the glass frit. The high temperatures required during the heating process damage the emitters so as to degrade the cathode. High temperature processes induce stress in the surfaces of the faceplate and the backplate due to temperature non-uniformities. Moreover, the high temperatures and the time at temperature increase the volume of gas outgassed from the materials used in the flat panel display.
Some prior art approaches attempt to reduce outgassing in prior art flat panel display fabrication processes by the use of materials that have a low outgassing rate and that have a low vapor pressure. Thus, only metals, glasses, ceramics, and select specially processed polymers are typically used within flat panel displays. These materials are typically processed by baking (at several hundred degrees centigrade) and are electronically or otherwise scrubbed in order to remove adhered molecules. However, only some of the outgassing is eliminated by such processes. Typically, a getter is used to minimize damage resulting from outgassing. The getter absorbs some of the chemicals released by outgassing. However, the getter only absorbs some of the outgassed particles, allowing the remainder of the damaging outgassed particles to possibly interact with the active surfaces of the flat panel display. The outgassed contaminates degrade the emitter surface causing electron emissions to be temporally unstable and to be generally reduced. In addition, ions formed through the collision of electrons with gas molecules can be accelerated into the emitter tips and may degrade their emission. Plasma formed in the same manner can short emitter tips to the overlying gate and can cause arcing at high field regions in the display. Thus, outgassing interferes with the operation of the cathode, resulting in reduced picture quality.
Alternate prior art heating methods for forming a seal between the faceplate and the backplate include the use of lasers that are focused on the glass frit. Typically, such methods heat the glass frit to temperatures of more than 400 degrees centigrade. However, since the heat is localized, the damage to the active areas is reduced. Damage resulting from oxidation is typically reduced by performing the heating process in an inert gas environment such as nitrogen). However, in order to prevent the glass of the faceplate and the backplate from cracking or breaking from the sudden temperature increase, the flat panel display must be pre-heated in an oven to the seal material glass transition temperature (typically 300 to 325 degrees centigrade).
In one prior art laser-sealing process, frit is disposed on the faceplate such that a gap is formed between the top of the frit and the bottom of the backplate. This gap is typically about one to two mils. The flat panel display assembly is then aligned and tacked so as to hold the faceplate and the backplate in their proper alignment. Typically, four tacks are used, one in each corner of the flat panel display assembly. A laser is then used to melt the frit. The heat of the laser melts the frit locally and causes the frit to expand such that the frit contacts the backplate, wetting the surface of the backplate and forming a "bead". The laser is moved, drawing the bead around the surface of the frit until the desired seal is formed. However, as the bead moves across the area to be sealed, friction from the movement of the bead can cause misalignment between the faceplate and the backplate, causing reduced image quality (sometimes resulting in a defective product). Additionally, the movement of the bead can cause overall stress across the entire flat panel display.
In laser heating processes, stress fracturing results from the torsional forces due to the moving bead and the cooler glass. More particularly, as the glass frit reaches a molten state, the glass plate of the faceplate is still relatively cold (though there is a small amount of radiative heating from the hot frit surface). When the molten glass frit surface touches the relatively cold glass, the quality of the joint suffers from the cooling of the joint surface by the glass of the faceplate. Stress fracturing reduces the life expectancy of the resulting product (in some cases, life expectancy is reduced to as little as few weeks). In order to reduce stress fracturing, additional processing steps are required. One such additional processing step consists of a pre-heating step that heats the flat panel display assembly to the required temperature. Such pre-heating can result in cathode degradation, stress the surfaces of the faceplate and the backplate, and cause outgassing.
Flat panel display fabrication processes are expensive and time consuming due in large part to the number of complex steps required in the bonding process. For example, when a two-step laser-heating process is used to seal the glass frit first to the faceplate and then to the backplate, the process typically takes approximately thirty minutes for a five inch square substrate (15 minutes for each side). Moreover, the outgassing and heat generated defects decrease yield and increase overall manufacturing cost. In addition, the numerous process steps take up a significant amount of time so as to cause low throughput rates.
Thus, a need exists for a flat panel display and a method for forming a flat panel display that will increase yield and throughput of manufacturing. A further need exists for a flat panel display and a method for forming a flat panel display that does not damage the active areas during the bonding process. In particular, a need exists for a flat panel display and a method for forming a flat panel display that minimizes outgassing and residual stress. A further need exists for a flat panel display and a method for forming a flat panel display which minimizes fabrication process time and which reduces manufacturing cost. The present invention meets the above needs.